Batteries such as lithium ion batteries installed in numerous electronic devices are required to have compact size and light weight in response to the increasingly high levels of portability and mobility of these devices accompanying reductions in size and weight. The reductions in battery size and weight required by electronic devices such as smartphones in particular require leading-edge specifications.
The battery cases of lithium ion batteries currently available for use in smartphones use a can-shaped aluminum thin sheet or aluminum foil laminated with a resin film. Resin film-laminated aluminum foil is frequently used for the purpose of improving capacity density per volume in particular. More recently, even thinner cladding materials are being required for the purpose of further reducing size and weight. However, when the thickness of the aluminum foil serving as the base material is reduced, problems occur such as increased susceptibility to the occurrence of pinholes during the course of production, being unable to ensure moisture impermeability, reductions in puncture strength and rigidity, and being unable to ensure adequate strength with respect to external impacts and internal expansion of the battery. Consequently, aluminum foil is considered to have reached its limit with respect to further reductions in thickness.
Therefore, attention has been focused on the use of foil made of stainless steel (stainless steel foil) since it demonstrates higher rigidity and strength than aluminum. However, since stainless steel has higher specific gravity than aluminum, in order to apply stainless steel to battery cases required for use in increasingly compact and lightweight electronic devices, stainless steel foil is required that has an extremely thin sheet thickness (such as 60 μm or less). In order to increase battery capacity in particular, stainless steel foil is required that it should enable high processability in terms of, for example, demonstrating uniform formability when formed into the shape of a square can despite having a sheet thickness of 60 μm or less.
Stainless steel foil having a thickness of 25 μm or less is disclosed in Patent Document 1 as an example of ultrathin stainless steel foil. When the thickness of stainless steel is made to be extremely thin, voids form accompanying the formation of cracks extending from etched edges in the direction of rolling. The invention disclosed in Patent Document 1 solves this problem by limiting the number of inclusions that measure 5 μm or larger in size.
In addition, examples of applying stainless steel foil to a battery case are disclosed in Patent Documents 2 to 4. Patent Document 2 discloses an example of a battery outer jacket member formed by pressing stainless steel foil having a thickness of 20 μm to 100 μm, Patent Document 3 discloses that an example of a battery jacket material formed by pressing stainless steel foil having a thickness of 100 μm, and Patent Document 4 discloses that an example of a battery jacket material formed by pressing stainless steel foil having a thickness of 40 μm to 150 μm.